1. Field of the Invention
The present invention relates to an exposure apparatus and its control method and, more particularly, to an exposure apparatus which measures focus errors on a substrate such as a mask substrate, semiconductor wafer, and the like used in the manufacture of devices such as a semiconductor device, liquid crystal display device, and the like, and tilt errors on the substrate, and corrects these errors, and its control method.
2. Description of the Related Art
In a conventional apparatus of this type, in order to match the surface of a wafer with the imaging plane of a projection optical system, any focus and tilt errors of the wafer are measured by a focus & tilt sensor, and alignment is made using a stage that can adjust such focus and tilt errors. In this case, the focus and tilt errors are measured a predetermined period of time after an X-Y stage is driven in a direction perpendicular to the optical axis to start alignment, and the same timing is used for all the positions in the wafer plane.
As described above, since the measurement start timing of focus and tilt errors is the same for all the positions in the wafer plane, high adjustment precision of focus and tilt errors and short alignment time cannot both be achieved at the same time. If the measurement start timing is advanced to shorten the alignment time, measurements must be started before vibrations of the stage in the optical axis direction settle down, and focus and tilt errors cannot be accurately measured, thus impairing the adjustment precision. By contrast, if the measurement start timing is delayed until vibrations of the stage settle down so as to improve the adjustment precision, the alignment time is prolonged. Since vibrations of the stage differ depending on different positions in the wafer plane, optimal measurement timings should be determined depending on different positions in the wafer plane. However, since the same timing is used in the conventional apparatus, the measurement precision of focus and tilt errors suffers, or the alignment time is prolonged.